In the manufacture of many precision products, it is necessary to lay out points and lines in precise relationship to reference planes or fiducial points. For example, the jigs and fixtures used to manufacture diverse products ranging from huge aircraft to microminiature electronics must be precisely located with respect to some reference point or plane. Typically, such points and surfaces are mapped out optically in relationship to a fixed fiducial mark established by an optical tooling target. The nature and sophistication of the optical target varies with the application. The optical target may be as simple as a hole drilled thru an I-beam or as sophisticated as an automatic alignment fiducial built into the integrated circuit photomask of modern optical microlithography. Once a primary fiducial mark is established, all measurements are made with respect to that primary fiducial.
In order to be effective, the fiducial must have adequate brightness and contrast to visually locate and distinguish the location. Moreover, the location of the fiducial mark must be certifiable relative to some significant point or plane in the work.
To enhance the visibility of a fiducial mark, optical targets have been provided with self-contained lighting systems wherein lights are positioned behind fudicial cross lines for illumination. One prior art optical target arrangement, for example, consists of a circular transparent disc with a black pattern fiducial on the front surface. The black pattern is illuminated from the rear by a small electric lamp. Due in large part to the space requirements of the lighting source, optical targets with self-contained illuminating systems are relatively large and present serious limitations when the target must be positioned in a limited space, as for example, when the tooling target must be placed between a surface and the objective lens of a pointing, depth measuring microscope.
Front lighting of optical targets has been attempted in the prior art. However, classical front lighting techniques yield reduced optical contrast between the object being viewed and its background, and provides excessive brightness variation within the image due to spectral flares and shadows from surface imperfections. The fiducial of the tooling target, in other words, cannot be readily resolved from its background.
There are similar shortcomings in providing a target that may be advantageously used as a component of a novel optical spherometer to optically determine the compound curvature of an unknown surface. Traditionally, the unknown curvature of large, opaque diffuse surfaces are measured by mechanical spherometers. A mechanical spherometer usually includes three depending parallel legs defining a plane. A parallel micrometer screw is threadably advanceable and retractable at a known and fixed position between the legs. Once the three legs are rested on a work surface to be measured, the micrometer screw is moved until it contacts the work surface. The sagittal height or the distance between the defined plane and the contact point is thus measured by the micrometer screw, and this measurement is used through simple calculations to determine the curvature of the work surface.
In applications where only limited space is available, as for example between a work surface and a pointing microscope, the physical size of mechanical spherometers makes their use impractical. Furthermore, mechanical spherometers require contact between the micrometer screw and the center of the measured work surface with a force sufficient to be reproduceable. Regulation of this contact force is normally done by the "feel" of an operator and is very difficult to control and reproduce. In addition to introducing error into the sagittal height measurement, physical contact by the micrometer screw may damage the surface being measured. Moreover, for some applications, it is necessary to align the center line of a spherometer with the center line of an optical system very precisely, as for example within a few ten thousanths of an inch. Such precise alignment is beyond the capacity of any known mechanical spherometer.
In U.S. Pat. No. 3,180,216 to Osterberg, a variable phase microscope is disclosed for enhancing the contrast of an image being examined. The apparatus, which operates by rotation of a deviated light vector to obtain destructive interference with a transmitted light vector over the image of a particle, thereby enhancing the contrast between the particle and its surround, is complex. A laser is required and a plurality of coherent light beams are needed to illuminate the specimen. A multiapertured, opaque disc is utilized.
A frontally illuminated specimen is inspected in a system disclosed in U.S. Pat. No. 2,318,705 to Morgan. This patent teaches the use of polarizing elements, to filter reflected light from the objective of a microscope examining the specimen, in order to reduce fogging of the image. The disclosure does not, however, teach either a method or an apparatus for imaging the specimen against a bright background with frontal illumination. Rather, light reflected by an objective is illiminated by a polarizing screen, while light passing through the objective to the specimen is passed by the polarizing screen.
In U.S. Pat. No. 3,759,618 to Rogers et al, a method and apparatus is disclosed for determining the position of an optical grid by interpreting relative movement between two grating systems with moire technology. The shadow of a grating is continuously cast onto a single photoelectric sensor through a plurality of subsections of a composite grid. Each of the subsections have the same line repeat dimension and orientation as the grating, but the lines are progressively shifted relative to the other subsections by chosen fractions of the line repeat dimension. The intensity of the grating shadow incident upon each of the grid subsections is then varied sinusoidally by alternating the polarization and direction and then providing selection filters over each of the different subsections so that respective maxima and minima of the sinusoidal variation at the various grid sections are then equally displaced. The output signal from the photoelectric sensor is then compared with a reference signal fluctuating sinusoidally in synchronism with the shadow intensity variation cycle.